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Index on TEXT versus CHAR(32)... fast exact TEXT matching

P: n/a
I have a table that stores TEXT information. I need query this table to
find *exact* matches to the TEXT... no regular expressions, no LIKE
queries, etc. The TEXT could be from 1 to 10000+ characters in length,
quite variable. If it matters, the TEXT may contain UNICODE characters...

Example:
CREATE TABLE a (id SERIAL, thetext TEXT);
SELECT id FROM a WHERE thetext='Some other text';

One way I thought to optimize this process would be to store an MD5 hash
of the "thetext" column and query on that:

CREATE TABLE a (id SERIAL, thetext TEXT, thetext_md5 CHAR(32));
SELECT id FROM a WHERE thetext_m5d=md5('Some other text');

Now, obviously I would want to build an INDEX on either "thetext" or
"thetext_md5", depending on which way I decide to make the table.

My question is, what is the absolute fastest way to find the exact match
to a TEXT column? Any amount of pre-processing is fine (such as
calculating the MD5's of all the TEXT tuples), but the query must be
extremely fast.

Has anyone compared (theoretical or practical) the performance of
querying a TEXT-based INDEX versus a CHAR(32)-based INDEX? Is my MD5
idea a waste of time? Is there something better than MD5? Would it be
better to store the "fingerprint" of the TEXT as an integer somehow, so
that the INDEX could be based on a INT* column?

Thanks for any help!

-Jon

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Jon Lapham <la****@jandr.org> Rio de Janeiro, Brasil
Personal: http://www.jandr.org/
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TIP 5: Have you checked our extensive FAQ?

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Nov 23 '05 #1
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2 Replies


P: n/a
> CREATE TABLE a (id SERIAL, thetext TEXT, thetext_md5 CHAR(32));
SELECT id FROM a WHERE thetext_m5d=md5('Some other text');


From your explanations :
- you need an exact match text->id
- you already know the text

Then, why bother with the TEXT column ?
I have no idea on the performance of pg text indexes, but I do know that
a table which fits in RAM, or an index which fits in RAM, is faster than
when it does not.

Try this :

create table my_searches (
id serial primary key,
text_md5 char[16] (or even a binary string)
);

Then create a table my_text (id,full_text) with appropriate foreign key
(to get a 1<->1 relationship between your tables) because you may still
want to remember the text. Index it on id of course.

My point is that your search table no longer has the big text field, it
has only constant size fields, (thus constant rowsize) and it is much
smaller. The index will also be smaller and maybe fit in RAM. You then :

SELECT id FROM my_text WHERE id in (SELECT id FROM my_searches WHERE
text_md5 = (your md5)) AND full_text=(your text);

Unless a MD5 collision occurs, which should be rare, the subquery will
return 1 result and the first SELECT will just do an index lookup on ID.

Now, ok, I'm stupid, I reimplemented the Hash index...

I'd be interested in your benchmarks of your real data with :
- table with full text + id, with btree index on text
- table with full text + id, with hash index on text
- the scheme I just came up with

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TIP 4: Don't 'kill -9' the postmaster

Nov 23 '05 #2

P: n/a
what about to use a CRC32 checksum of the text, computed by client
application an then make index of crc32 data ?

ie

add column crc int4,
add column md5 varchar(255 )
create index blabla on mytable (crc)
or even create index blabla2 on mytable (crc,md5)

and query like ......... where crc='crc32 of your searched text' AND
md5='md5 hash'

i am not shure, but this should work pretty fast.
----- Original Message -----
From: "Jon Lapham" <la****@jandr.org>
To: <pg***********@postgresql.org>
Sent: Saturday, September 04, 2004 3:04 PM
Subject: [GENERAL] Index on TEXT versus CHAR(32)... fast exact TEXT matching

I have a table that stores TEXT information. I need query this table to
find *exact* matches to the TEXT... no regular expressions, no LIKE
queries, etc. The TEXT could be from 1 to 10000+ characters in length,
quite variable. If it matters, the TEXT may contain UNICODE characters...

Example:
CREATE TABLE a (id SERIAL, thetext TEXT);
SELECT id FROM a WHERE thetext='Some other text';

One way I thought to optimize this process would be to store an MD5 hash
of the "thetext" column and query on that:

CREATE TABLE a (id SERIAL, thetext TEXT, thetext_md5 CHAR(32));
SELECT id FROM a WHERE thetext_m5d=md5('Some other text');

Now, obviously I would want to build an INDEX on either "thetext" or
"thetext_md5", depending on which way I decide to make the table.

My question is, what is the absolute fastest way to find the exact match
to a TEXT column? Any amount of pre-processing is fine (such as
calculating the MD5's of all the TEXT tuples), but the query must be
extremely fast.

Has anyone compared (theoretical or practical) the performance of
querying a TEXT-based INDEX versus a CHAR(32)-based INDEX? Is my MD5
idea a waste of time? Is there something better than MD5? Would it be
better to store the "fingerprint" of the TEXT as an integer somehow, so
that the INDEX could be based on a INT* column?

Thanks for any help!

-Jon

--
-**-*-*---*-*---*-*---*-----*-*-----*---*-*---*-----*-----*-*-----*---
Jon Lapham <la****@jandr.org> Rio de Janeiro, Brasil
Personal: http://www.jandr.org/
***-*--*----*-------*------------*--------------------*---------------
---------------------------(end of broadcast)---------------------------
TIP 5: Have you checked our extensive FAQ?

http://www.postgresql.org/docs/faqs/FAQ.html

---------------------------(end of broadcast)---------------------------
TIP 9: the planner will ignore your desire to choose an index scan if your
joining column's datatypes do not match

Nov 23 '05 #3

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